Several DMI fungicides can be effective for preventive control of fairy ring on creeping bentgrass greens.

Fairy rings are a severe disease problem on golf courses and other highly maintained turf areas. Symptoms appear along the outer margin of a developing subsurface fungal colony, where the density of mycelium is greatest. Rings can exhibit three types of symptoms, all of which can be observed in an affected area at the same time (7). The most severe symptom (type I) leaves necrotic bands of turf from 4 to 12 inches (10 to 30 centimeters) wide and up to 15 to 30 feet (4.5 to 9 meters) in diameter. These necrotic bands are most commonly an artifact of drought-stressed turf, caused by a combination of dense fungal mycelium and the production of organic acids that coat sand particles and render the underlying soil hydrophobic.

A second symptom of fairy ring is the stimulation of lush green turf growth in rings or arcs (type II) caused by release of plant-available nitrogen. Ammonia levels may reach toxic levels, contributing to plant mortality (2,8). Fairy rings may also produce circles of basidiocarps (type III), which have no direct effect on turf health but can
negatively affect appearance and playability.

Nearly 60 different basidiomycete fungi have been implicated in fairy ring occurrence (1). However, the fungus responsible for a fairy ring outbreak on a green is usually unknown because routine mowing inhibits formation of the mature basidiocarp (mushroom or puffball) required for traditional identification. Marasmius oreades is the most researched and most commonly named causal agent of fairy ring damage. However, in a recent study, Arachnion album Schwein., Bovista dermoxantha (Vittad.) De Toni, (= Lycoperdon dermoxanthum Vittad.), and Vascellum curtisii (Berk.) Kreisel (= L. curtisii Berk.) were more commonly observed in association with type I and type II fairy ring symptoms on sand-based greens in the southeastern United States (5). The different fungi involved could differ in their sensitivity to fungicides, resulting in inconsistencies in product performance across locations.

Fairy ring control with fungicides

Fungicides that target basidiomycetes, such as flutolanil (ProStar, Bayer), azoxystrobin (Heritage, Syngenta) and pyraclostrobin (Insignia, BASF) have been found most effective for curative fairy ring control (Table 1), but curative suppression with these fungicides has been only marginally efficient because of the water-repellent nature of the infested soil. Therefore, for curative applications, mixing fungicides with soil surfactants and drenching with water are necessary to deliver the fungicide to the target zone (3,4). Even with these methods for fungicide delivery, turfgrass recovery from fairy ring injury is slow because both pathogen suppression and remediation
of soil physical properties are necessary.

Despite the problems associated with curative fungicide applications, a preventive fungicide program for fairy ring control has not been investigated fully. Preventive fungicide applications are a standard control strategy for many soil-borne turfgrass diseases such as spring dead spot, summer patch and take-all patch. Earlier research (9) and superintendent observations have indicated that triadimefon (Bayleton, Bayer) applications made in the spring may provide residual fairy ring control.

The purpose of this research was to investigate the influence of various application strategies on the performance of DMI fungicides for preventive fairy ring control. The specific objectives were to evaluate the impact of single preventive application, timing and fungicide rate on fairy ring control, and to investigate the influence of irrigation timing and soil-surfactant tank mixtures on preventive fungicide performance.

Materials and methods

Site description and data analysis

Field experiments were conducted at the Lake Wheeler Turfgrass Field Laboratory in Raleigh, N.C., on a Penn A-1 creeping bentgrass (Agrostis palustris Huds.) research green. The plot was constructed in 2005 according to USGA recommendations with a root-zone mix of 85% sand and 15% sphagnum peat moss by volume. In 2007, the plot area was found to be naturally infested with fairy rings caused by Vascellum curtisii; however, Arachnion album was also identified from infested soil and basidiocarps in 2009 (5).

The green was fertilized with nitrogen at 283 pounds/acre (317 kilograms/hectare) in 2007, 327 pounds/acre (366 kilograms/hectare) in 2008, and 261 pounds/acre (292 kilograms/hectare) in 2009. Plots were mowed five times per week at 0.138 inch (3.5 millimeters) in the fall and spring and at 0.157 to 0.177 inch (4-4.5 millimeters) in summer. The experimental area was aerified in the spring and fall with 0.63-inch-diameter (1.6-centimeter) tines set at 1.5-inch to 2-inch (3.8- to 5.1-centimeter) spacing. Fungicides not known to affect fairy ring, such as chlorothalonil (Daconil Ultrex, Syngenta), fosetyl-Al (Chipco Signature, Bayer), iprodione (Chipco 26GT, Bayer) and boscalid (Emerald, BASF) were applied for prevention of common foliar diseases. The study site was irrigated as needed to prevent drought stress.

Disease severity was evaluated every seven to 14 days from May through September. Disease was assessed by measuring the length and width of fairy ring arcs to derive percent diseased plot area.

Area under the disease progress curve (AUDPC) was calculated. The date of initial disease observation was also recorded and analyzed to assess treatment impact on disease incidence. After each fungicide application and during each disease severity rating, turfgrass quality was assessed visually and rated on a scale of 1-9, where 9 is best, 5 is minimally acceptable and 1 is bare ground. Data were analyzed to determine treatment effects among and within trial years.

Experiment 1: Optimal application rate and timing

A three-year field experiment (experiment 1) was initiated in 2007 to determine the optimal rate and soil temperature-based timing of triadimefon (Bayleton 50DF, Bayer) and tebuconazole applications for fairy ring prevention. (Editor’s note: The tebuconazole product tested was a previous experimental formulation that was not released.) Plots were 5 feet × 10 feet (1.5 meters × 3 meters), with treatments replicated four times in a split-block randomized complete block design with application timing as the main plot and fungicide as the subplot. Plots were not re-randomized between trial years in an effort to assess the multiyear impact of the treatments.

A two-year field experiment (experiment 2) was initiated in 2008 to examine the impact of irrigation timing and fungicide + surfactant tank mixtures on the performance of preventive DMI applications. Plots were 5 feet × 10 feet, with treatments
replicated four times in a split-block randomized complete block design with fungicide as the main plot and irrigation timing and surfactant tank mix as subplots. Plots were not re-randomized between trial years in an effort to assess the multiyear impact of the treatments.

Fungicide treatments included two applications of a low rate of triadimefon (Bayleton FLO at 1 fluid ounce/1,000 square feet [0.15 gram a.i./square meter]) and triticonazole (Triton 70WG at 0.25 ounce/1,000 square feet [0.08 gram a.i./square meter]) and an untreated control. Fungicides were applied as described previously with or without the surfactant Revolution (Aquatrols) at a rate of 6.0 fluid ounces/1,000 square feet (1.86 milliliters/square meter). No additional surfactants were applied to these plots during the experiment. Irrigation timing treatments included hand watering with 0.25 inch (6.4 millimeters) of irrigation either immediately or 10 hours after fungicide application.

Treatments were applied on March 28 and April 25, 2008, and March 31 and April 28, 2009. The five-day average soil temperature at 2 inches (5 centimeters) on the first application date of both years was 55 F-60 F (13 C-16 C).

Results

In 2007, type I and II symptoms were first observed in late May just before the 75 F (23 C) application, making this a curative treatment in eight of the 20 total plots. Type II fairy ring symptoms developed in the experimental areas in mid-June 2008 and mid-July 2009, and were not present in the experimental area during the treatment
periods. In both field experiments, disease severity decreased over the years of the study. In experiment 1, AUDPC values were equivalent in 2007 and 2008 and significantly decreased in 2009. In experiment 2, AUDPC values also significantly decreased in 2009.

Grayish brown turf discoloration was observed in plots treated with Bayleton 50DF or Bayleton FLO in late May 2008, resulting in a decrease in overall turf quality compared with plots treated with tebuconazole or Triton. This effect was more pronounced in plots treated twice with Bayleton FLO + Revolution than in plots treated with Bayleton FLO alone. All Bayleton-treated plots recovered by mid-June, and overall turf quality ratings did not fall below acceptable levels (less than 5).

Field experiment 1: application rate and timing

Plots treated with both rates of tebuconazole and the high rate of Bayleton 50DF had lower disease severity in 2007 and 2008 compared with the untreated control (Figure 1). The low rate of Bayleton 50DF reduced disease severity in both years but was not statistically different from the control in 2007. AUDPC values tended to be lower in plots sprayed at the 55 F and 60 F soil temperature thresholds, but were not statistically different from any other application timing.

Field experiment 2: surfactant tank mix and irrigation timing

Plots treated with two low-rate applications of Bayleton FLO or Triton 70WG had significantly lower AUDPC values than the untreated controls in both 2008 and 2009, and the onset of symptoms was significantly delayed in 2008. No differences were detected in the level of control afforded by either fungicide. Irrigation timing did not affect the efficacy of fungicide applications in either year (Figure 2).

Although the differences were not statistically significant in either year, AUDPC values were higher in plots that received a fungicide + surfactant tank mix as opposed to fungicide alone (Figure 3). In 2009, fairy ring symptoms were observed significantly earlier in plots treated with a fungicide + surfactant tank mix than in plots that received only a fungicide. Because symptoms of localized dry spot developed in both years, plots not treated with a surfactant had significantly lower turf quality than plots that received a surfactant.

Discussion

This study demonstrates the efficacy of using certain DMI fungicides for preventive fairy ring control, thereby reducing the need for more numerous and costly curative fungicide applications and soil remediation procedures. No differences in field efficacy were detected among the DMI fungicides tested in this study; however, two spring applications on a 28-day interval tend to provide longer residual control under high disease pressure than a single application.

Application timing

Proper timing of preventive applications is necessary to maximize the residual effectiveness of the fungicide and target a vulnerable portion of the pathogen’s life cycle. Because preventive applications are made before plant symptoms are evident, environmental cues, such as soil temperature, have been used as indicators of application timing for control of soil-borne turf pathogens. In this field study, we were not able to statistically determine the most effective soil temperature threshold for fairy ring prevention. A weak data trend suggests that preventive applications made at five-day soil temperature averages of 55 F or 60 F (13 C or 16 C) may be most effective, but further study should be conducted to confirm this result.

Surfactant tank-mix and irrigation timing

Fungicide application methods may affect the efficacy and duration of preventive control. Other surfactant chemistries may respond differently, but data in this research suggest that tank-mixing a surfactant with a preventive fungicide does not increase disease control and may result in slight phytotoxicity. In a curative situation, however, fungicides must be tank-mixed with a surfactant to aid delivery of the material through the hydrophobic layer and into the target zone of the soil profile. Because hydrophobicity is not present in a preventive situation, the surfactant may diminish the soil residual of the fungicide and, therefore, reduce its efficacy.

Results of this study also indicate that the impact of watering-in the preventive DMI applications immediately versus 10 hours later is negligible. Superintendents should therefore be able to wait until the night irrigation cycle to water-in the fungicide if play is a concern. This statement comes with a degree of caution, however, as inadequate control has been reported by superintendents if the fungicide is not watered-in within a 10- to 12-hour time frame.

Potential phytotoxicity

DMI fungicides often exhibit plant-growth-regulating effects and may cause phytotoxicity in some turfgrasses, particularly when applied during high temperatures. Applying DMI fungicides in the spring, watering them in, and using lower rates should diminish phytotoxicity. In a high temperature period in late May 2008 (maximum air temperatures greater than 90 F [greater than 32 C]), we observed slight phytotoxicity associated with preventive Bayleton applications that affected the color and quality of creeping bentgrass. This effect occurred in only one of the three years of the study and was short-lived, but should still be considered. Future research should also assess the impact of preventive DMI applications on spring green-up of bermudagrass, and the transition from a cool-season turf species used during the winter months to a warm-season turf species.

The research says

Preventive fungicide applications may be more effective than curative applications for managing fairy ring on putting greens.

Plots treated with one or two applications of Bayleton, Triton or tebuconazole in the spring had significantly less fairy ring occurrence than untreated plots. This result
concurs with other research that also demonstrates control with two low-rate applications of Eagle, Tourney or Trinity.

The first preventive application should be made in spring when the five-day average soil temperature at a 2-inch depth reaches a 55 F-60 F threshold.

Tank-mixing a surfactant with a preventive spring fungicide application did not increase fairy ring control; applying surfactants aimed at localized dry spot on a regular schedule, but not tank-mixed with a preventive fairy ring application, may be most effective.

Watering-in the preventive DMI fungicide immediately rather than 10 hours after application did not affect the effectiveness of fairy ring control.

Future research

Results from this and an earlier study (6) indicate that preventive low-rate applications of the DMI fungicides triadimefon, triticonazole, tebuconazole, metconazole or myclobutanil are an effective tool in the suppression of fairy ring on putting greens caused by either Bovista dermoxantha or Vascellum curtisii. These results are specific to sand-based putting greens and have not yet been evaluated for tees, fairways or other areas. For this reason, effectiveness may vary for fairy ring suppression caused by another species or for another soil type.

There is a high level of specificity to the observations made in this study, which centers on the causal agent and ecosystem and not a broadly defined plant symptom. Future research regarding fairy ring control strategies should also use recently outlined methods (5) to identify the pathogen being investigated. This would allow for direct comparison with this and other fairy ring management research, and facilitate control recommendations that target the specific pathogen.

Funding

This research was supported by grants from Bayer Environmental
Science, Carolinas Association of Golf Course Superintendents,
the Center for Turfgrass Environmental Research and
Education at North Carolina State University and the Environmental
Institute for Golf.

Acknowledgments

We thank E. Rosebrough and A. Rosebrough (North Carolina State University) for valuable technical assistance. This article is based on an earlier article by the same authors that appeared in the July 2012 issue of the journal Plant Disease: Evaluation of preventive fungicide applications for fairy ring control in golf putting greens and in vitro sensitivity of fairy ring species to fungicides. Plant Disease 96(7):1001-1007(http://dx.doi.org/10.1094/PDIS-09-11-0779-RE).

Gerald (Lee) Miller (turfpath@missouri.edu) was a graduate student and Lane P. Tredway was an associate professor in the department of plant pathology, North Carolina State University, Raleigh, N.C., at the time this research took place. Michael D. Soika is a laboratory research specialist in the department of plant pathology, North Carolina State University.